Microbes played a significant role in human evolution, study says

Chuck Bednar for redOrbit.com – Your Universe Online

Human children remain dependent on their parents far longer than other animals, and elderly people live a comparably long time after their reproductive cycles have come to an end.

What causes this unusual phenomenon? Researchers from New York University (NYU) and Vanderbilt University believe that a person’s microbiome, or the bacteria and other organisms making their home inside his or her body, could be the cause of these uniquely human traits.

Martin Blaser, professor of medicine and microbiology at the NYU Langone Medical Center, and Vanderbilt mathematics professor Glenn Webb report their findings in research published online Tuesday in mBio, the official journal of the American Society for Microbiology.

Every living thing, whether plant or animal, is home to a vast and distinctive group of microorganisms. In fact, there are approximately 100 trillion microbial cells in the human microbiome, and those organisms outnumber actual human cells by a 10:1 ration, the study authors explained.

It had long been thought that these microorganisms had an extremely limited impact on their human hosts, but recent studies have shown that their sphere of influence involves more than just assisting with digesting food and producing bodily odors. Our microbiomes also assist in the development of our brain, help our reproductive systems and protect us against infections.

These recent findings have led to the development of the hologenomic theory of evolution, which submits that the object of Darwinian natural selection is not just the individual organism, but the organism and its associated microbial community as well.

Blaser said that the idea that microbes had a significant impact on human age structure came from his longtime analysis of Helicobacter pylori, a bacterium found in the stomachs of half of the global population. H. pylori typically co-exists peacefully in a person’s stomach, and can even help regulate stomach acid levels, but is also one of the primary causes of stomach cancer.

“I began thinking that a real symbiont is an organism that keeps you alive when you are young and kills you when you are old. That’s not particularly good for you, but it’s good for the species,” Blaser explained. He recruited Webb to develop a mathematical model capable of describing dynamic biological processes in order to test this concept.

The duo decided to develop a model of an early hunter-gather population to analyze the role which the microbiome might have played during their lives. They divided their simulations into three different age groups – juvenile, reproductive and senescent – and looked at how the population would respond to different combinations of fertility or mortality rates.

This enabled they to develop a baseline case using the best estimates of these rates they could find. They then added the risk of mortality based on various microbial profiles, including one based on the diarrhea-causing bacteria Shigella that increased mortality among children, and another involving an H. pylori-type mortality factor that increased with age.

Blaser and Webb found that this decreased the percentage of the senescent population, which ultimately helped the younger populations by reducing the elderly’s demand on food and other resources. The end result, they said, was stronger-than-baseline population growth and stability – findings consistent with the belief that evolution may have influenced the human microbiome in order to favor bacteria that target the aging.

The study authors went on to double the fertility rate in one simulation, and found that the population became unstable and experienced “catastrophic boom-bust cycles” in response to events that caused major population loss. Another variation found that even slight increases in the elderly population caused the hunter-gatherer population to begin declining.

“In addition to providing validation to the proposition that the microbiome may be shaping the human age structure, Webb observed that the modeling effort also reveals an underlying truth about human population growth,” officials from Vanderbilt University explained. “We have the right fertility and mortality rates to support our unusual age structure.”

“If you go back 30,000 to 40,000 years ago, there were only 30,000 to 40,000 people in the world and they were scattered over Africa, Europe and parts of Asia,” added Webb. “Are we lucky just to be here? Or did we survive because our ancestors were robust enough to handle all the environmental changes and natural disasters they encountered? According to our equations, it was because they were robust enough.”

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